Revert "change district owner if defending units are <= 0"

This reverts commit e42694add4b80c12ad96a6421fbf1711e140cc42.

The solution this commit proposes will not work because the code it changes is invoked every time the report of an attack is shown. It should also be invoked at the time the result of the battle is calculated.
Bugfixes
District takeover

The attacker takes control over the defenders district if the defender has no units left
Session persistant change of current district

- Current district can be changed by clicking on a district on the map
- Stored in $_SESSION['currentdistrict']
- Used on Map (Attack button), Unit and Buildings page